Loss of Fragile X Mental Retardation Protein (FMRP) causes a developmental brain disorder characterized by impaired synaptic connectivity and disrupted activity-dependent modulation in the late developing brain. Loss of FMRP is associated with a range of neurodevelopmental disorders (NDDs) with symptoms including intellectual disability, autism and childhood epilepsy. Our laboratory established the powerful Drosophila disease model and showed human FMRP displays total functional conservation in this model. We have repeatedly proven this model provides direct insights into the molecular and cellular bases of the human disease state. In this revised competitive renewal proposal, I ask for your urgently needed support to allow us to continue to take advantage of this wonderful genetic system, and relative simplicity of the learning/memory neural circuit in the Drosophila brain and its thoroughly-characterized behavioral output, to test core hypotheses regarding FMRP loss and proposed interventions to correct the resultant developmental brain defects. Experimental approaches will target the well-defined Mushroom Body (MB) circuit, a brain center receiving input from multiple sensory modalities to mediate associative learning and memory consolidation. In the first aim, we will test the hypothesis that FMRP regulates the development of the appropriate excitatory (E) vs. inhibitory (I) synaptic balance within the MB circuit. We propose genetic and pharmacological means to correct E vs. I defects independently, to assay restoration of architectural, functional and behavioral output defects in the null mutant state. We hypothesize this will be a fruitful new avenue for therapeutic intervention. In the second aim, we examine the inter-dependence of synaptic activity and FMRP function in shaping MB circuit maturation. We hypothesize that synaptic activity regulates E vs. I synapse elimination (pruning) relative to stabilization via a FMRP-dependent mechanism. We will use a combination of transgenic activity blockers (e.g. tetanus toxin) and photocurrents (e.g. light-gated ion channels) in targeted E vs. I neurons in staged developmental trials, examining outcomes in controls compared to FMRP loss and gain-of-function mutants. In parallel, we will use transgenic [Ca2+] reporters to chart activity-dependent E vs. I changes during MB circuit development. In the third aim, we tackle the role of FMRP as a translational regulator controlling development stage appropriate protein synthesis during MB circuit maturation. We propose to characterize the brain proteome over the developmental time course we have established for FMRP function. Such desperately needed developmental profiling has never before been done in any disease model. Together, these aims are designed to make maximal use of the powerful and proven Drosophila disease model. My lab is the only lab poised to pursue this work, and I truly believe we can aid enormously in providing understanding and devising treatments for this most common heritable cause of cognitive dysfunction and autism spectrum disorder.